I've been trying to get Amazon to do that for a year, with no success. I've bugged my editor at Ace so many times that she's sick of hearing about it.
Apparently they are so backlogged they only do some books, and their criteria is obscure. Name authors usually get it done, but I've also seen quite a few debut authors in uf and paranormals that also manage to get that perk. Bantam Spectra seems to have an in on that.
It's really a shame, because with a new author, you never know what you're going to get. Even a few pages is often enough to tell if you'll like the book or not -- I'm hesitant to buy a book myself without being able to at least get a look at it unless I already know the author's work.
Of course, I prefer bookstores anyway, and have a great F/SF bookstore five minutes from my house, but not everyone is so fortunate.